1. Field of the Invention
This invention relates to a conveyor means which uses a linear motor for the manufacturing process of precision circuit parts in a clean room.
2. Prior Art
Conventionally, endless trolley conveyors of suspended type or the like are used widely as conveyor means, but trolley conveyors of this type adopt the system of slow starting-high speed transporting-slow stopping for the purpose of preventing package shaking and vibration. Therefore, this system requires reduction of the transporting speed to a large extent at the stages of slow starting and slow stopping, with the result of low transporting efficiency. In order to eliminate such disadvantage, conveyor means using a linear motor have recently been put to practical use. Also, in a clean room of a precision circuit parts manufacturing plant, conveyor means for transporting precision parts should have as small a mechnical contact portion as possible because there is a danger that microscopic dust is generated by parts in frictional contact. In this respect, a non-contact type linear motor system is most suitable.
For the conventional conveyor means using a linear motor, a method in which a transporting pallet loaded with goods for transportation is carried while it is kept lifted by compressed air jet from a transporting duct has been suggested to improve transporting efficiency. However, constant jetting of compressed air covering the whole length of a transporting course involves consumption of a large amount of compressed air. Moreover, in the case of an emergency stop of a transporting pallet during running, it is required to effect automatic return to the original point by inclining the entire transporting course to let a pallet fall by its own weight. In this case, however, there is a difficulty that if the total length of the transporting course is long, the difference in level between the starting point and the terminal point becomes large due to the inclination of the transporting course, with the result of poor transporting efficiency during normal transporting and the necessity of inclination of the operating device. Moreover, it has been found that due to the force of inertia at starting and at stopping of a transporting pallet, goods being transported in a container vibrate, with the result of damage to the goods, generation of foreign matter, adhesion of foreign matter to the goods and this raises a serious problem especially in the case of transporting a container in which many silicon wafers are enclosed in the manufacturing of semiconductor devices because the silicon wafer is composed of thin, brittle material.